When making red wine from grapes, you will be fermenting crushed fruit. So the skins, seeds, pulp, probably some stems and other debris are all mixed into your fermenting wine. When the yeast are active, all the carbon dioxide (CO2) they produce is rising to the surface pushing those solids up with it. They collect on the surface, forming a cap. The whole point of putting grape skins in your must is to extract beneficial compounds from them, and this can’t happen while they’re out of contact with the fermenting wine. Also, yeast have no access to these solids and can’t crowd out other microorganisms, leaving the cap open to infection.
You can see the solution in the above photo, where I’m physically submerging these solids with a long handled spoon. I keep doing this until all the solids have been pushed down into the fermenting wine. “Punch down the cap” like this twice a day, while the yeast are active, and you should have no problems with extraction or infection.
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